Development of agitation systems in biogas plants in order to optimize mixing-behaviour and to reduce energy demand with the help of Computational Fluid Dynamics (CFD) simulation

Status

completed

Summary

The purpose of this project is the investigation of the mixing-behaviour and performance of the agitation systems in biogas plants, and to present them through Computational Fluid Dynamics (CFD) simulation. This EDP-tool can model and predict the actual fluid dynamics in a closed system. Thus it may help to plan optimal reactor geometries, position and combination of agitation systems as well as to minimize the energy requirements.

Two already operating biogas plants with different types of agitation systems were investigated. Their process control system was used to collect and analyze the data.

The rheological behaviour of the reactor content was experimentally studied and described taking into account the different substrates fed to the plant and the resulting particle-size and fiber-length distributions as well as the viscosity. These experimental procedures were also improved and detailed on the basis of previous experience. For this purpose a new type of a viscosimeter was designed which is able to measure the complex rheological behaviour of the fermtenter slurries.

The CFD-models was tested through addition of Bacillus Globigii and interpretation of the resulting distribution curves. Modifications of these basic models were performed and thus predictions for the following problems may be stated:

• Optimization of the mixing-dynamics in existing plants.
• Optimal reactor geometries.
• Minimization of the energetic requirements of the agitation systems.
• Optimal agitation set-up for an agricultural biogas plant.

The project was divided into work packages during a period of two years. In two work packages the actual status of these two biogas plants were investigated. Other packages had the aim to investigate the slurries used in these plants. The emphasis was to find out detailed information about the rheological behaviour of the slurries. For this reason the macro-viscosimeter was designed, though other properties are a part of the research in this project like the influence of the substrates to density, particle-size, fiber length and content. All of them are related to the rheological behaviour.

All these results were used as boundary conditions and material properties in the CFD simulations. The work packages concerning CFD simulation contain single-phase and multiphase simulations and variation of process parameters like the rotation number of the paddle stirring devices.

Results and conclusions of the project

During the project, different kinds of biogas digesters and stirrers were simulated using CFD. The emphasis lied on single phase simulations. To define the power characteristics of the stirrers for two Austrian biogas plants in Reidling and Strem the rotation speed of the paddle stirrers were changed to 4 determined numbers, distributed over the whole range of the gears of the drive mechanisms. Using this method a optimized operation point could be found. For both plants multi-phase simulations were also implemented to study the mixture process. Future plants of the companies AAT and Thöni were investigated with single phase simulations to find the best configuration for their future plants.

All simulations show that it is not necessary to run the plants at the maximum available speed. Using propeller stirring devices increases the energy consumption, but not the average velocity in the digesters. The power consumption of the propellers can be decreased by at least about 10% if they are installed in flow direction. These results lead to the conclusion that the speed of the paddle stirrers should be reduced, and using propeller stirrer for longer periods should be avoided.

With these simple upgrades the energy consumption in Strem and Reidling could be decreased to about 30% of their actual value.

Project management

Technische Universität Wien

Authors

• DI Stefan Pohn, Dr. Michael Harasek
  Technische Universität Wien
• DI Ludek Kamarad, DI Günther Bochmann
  Universität für Bodenkultur

Project partners

• Universität für Bodenkultur
• AAT Abwasser- und Abfalltechnik GmbH
• Thöni Industriebetriebe GmbH
• Ing. Karl Pfiel GmbH
• Biogas Strem Errichtungs- und Betriebs GmbH & CO KG.

Ass.Prof.Dipl.-Ing. Dr. Michael Harasek
Technische Universität Wien
Institut für Verfahrenstechnik, Umwelttechnik und Technische Biowissenschaften
Thermische Verfahrenstechnik - Numerische Strömungssimulation
Getreidemarkt 9/166
A-1060 Wien
Tel.: +43 (1) 58801 166202
Fax: +43 1 58801 15999
E-Mail: michael.harasek@tuwien.ac.at
Web: www.vt.tuwien.ac.at